One way of handling stormwater runoff, such as from streets or a large shopping center parking lot, is to collect the water by means of surface drains, and send it to an underground structure, where it can be detained and subsequently released in a controlled manner to a water course or municipal storm sewer, and or so it percolates over time into the surrounding soil. Underground structures which have been used heretofore for the purpose comprise corrugated pipes, arch shape cross section molded chambers, concrete galleries and the like. Commonly, they are buried in a bed of stone or gravel, and with the stone interstices they provide void space within the earth, for receiving stormwater. For examples of apparatus used for receiving stormwater see Moore et al. Pat. No. 5,890,838, Maestro Pat. No. 6,361,248.
Stormwater typically carries with it debris, such as sand, paper and plastic things, and other miscellaneous matter, which had accumulated on the surface being drained. Catch basins and other devices are commonly used to trap or settle out such debris, to prevent large amounts from entering the subterranean system. For the debris that nonetheless enters and accumulates in the underground stormwater systems, access ports and manholes provide access for cleaning.
Increasingly, there are regulatory limits on the total suspended solids TSS content of stormwater which is discharged to streams or municipal storm sewer systems from parking lots and the like, and from stormwater detention systems used with such. But it is not so easy to prevent finer solids from entering the underground systems, or to retain them in the system in a way which enables them to be removed, so the system does not become gradually clogged. The prior art comprises various devices and methods, including Stever Pat. No. 6,350,374, which describes apparatus comprising a series of baffles; and, Stewart Pat. No. 5,322,629 which describes flowing the storm water through a layer of leaf compost material. Applicants are unaware of widespread use of such. So, there is a need for improvement in stormwater systems, to isolate or contain within a system debris which enters with stormwater, so it can be later largely removed, and so it is prevented from discharging to a watercourse.